1. Field of the Invention
The present invention relates to an active suspension in a vehicle such as an automobile, and more particularly to a fluid pressure type active suspension.
2. Description of the Prior Art
As shown in, for example, Japanese Publication 60-500662, it is known to incorporate in a vehicle such as an automobile an active suspension which comprises a fluid pressure type actuator provided between a vehicle body and a wheel, and a pressure control means for controlling the fluid pressure in said actuator, wherein the control of the pressure in said actuator refers as one of the control parameters to the rate of change of the vehicle height at a portion of the vehicle body corresponding to said wheel.
By such a fluid pressure type active suspension it is possible to more effectively control a transitional change of the posture of the vehicle body as compared with a similar suspension in which the rate of change of the vehicle height is not included in the control parameters.
However, since the rate of change of the vehicle height increases greatly when the vehicle runs on a rough road, the operation of the active suspension of the above-mentioned type reflecting the rate of change of the vehicle height in its control becomes excessive, and as a result a shock is apt to occur in the vehicle body, thereby affecting the comfortableness of the vehicle.
Particularly when the rate of change of the vehicle height fluctuates at a high frequency and/or a large amplitude, the fluid pressure in the actuator can not sufficiently follow the control output of the control means due to a delay in the response of the control system, and as a result a high vibration is apt to occur in the suspension and/or a noise would be generated due to a delay in the operational phase of the suspension, causing an increase of consumption of the working fluid.
Further, as shown in, for example, Japanese Publication 63-227411, it is also known to operate a similar active suspension in a manner that the fluid pressure in the actuator is controlled by using an acceleration of the vehicle body as one of the control parameters so that a transient change of the posture of the vehicle body is more effectively suppressed.
However, in such an active suspension, since the rate of change of the acceleration of the vehicle body increases greatly when the vehicle is turning or accelerated or decelerated on a rough road, the posture of the vehicle body is apt to become unstable, thereby also affecting the comfortableness of the vehicle. Further, when the vehicle body touches or bumps against a foreign body while it is running, the acceleration of the vehicle body is amplified, and therefore, a shock is apt to occur in the vehicle body or the posture of the vehicle body is apt to be abruptly changed.